Our laboratory has developed a model for transporting fatty acids in tissues by lateral movement in an interfacial continuum of external leaflets of intracellular and plasma membranes. When oleic acid is added to aqueous solutions, it covers the monolayer. We found that the spreading pressure of oleic acid increased from 34 to 44 dynes/cm when pH of the medium was increased from 5.4 to 7.4., demonstrating that affinity of fatty acids for lipid monolayers increased with pH. We studied the effect of pH on movement of oleic acid in a monolayer covering the surface of aqueous medium in a trough with interconnected compartments. When pH of the medium in one compartment was increased above that in the other compartments, fatty acid molecules in lenses above that compartment entered and moved in the monolayer to the other compartments where they formed lenses above the monolayer. We found, using freeze-fracture electron microscopy, areas free of protein particles in external leaflets of intracellular and plasma membranes in adipose tissue processed at pH 9.0. The surfaces of these areas sometimes formed multiple folds that abutted on myelin figures. We concluded that these areas were formed by fatty acids that entered leaflets from lipolyzed lipid droplets in adipocytes and spread throughout a continuum of leaflets, and when they overcrowded the coninuum, they produced foldings and lamellar extension of the continuum in the form of myelin figures. These findings support our model for transport of fatty acids in tissues.